Spark plug

Abstract

A spark plug has a metal shell and a ground electrode joined at a base end portion thereof to the metal shell and includes a surface layer and a core higher in thermal conductivity than the surface layer. The surface layer has a thickness of 0.2 to 0.4 mm at a specific position that is located 1 mm from the base end portion. The spark plug satisfies the following condition: W1≧W2×1.55−(W3+0.25) where W1 (mm) is a width of the metal shell at a weld region of the metal shell joined with the base end portion in a specific direction that extends perpendicular to the axis direction through a center line of the ground electrode; W2 (mm) is a thickness of the ground electrode at the specific position in the specific direction; and W3 (mm) is a thickness of the surface layer at the specific position in the specific direction.

Having described the invention, the following is claimed: 
     
       1. A spark plug, comprising:
 a rod-shaped center electrode extending in an axis direction of the spark plug; 
 an insulator having an axial hole formed in the axis direction so as to retain the center electrode in the axial hole; 
 a metal shell circumferentially surrounding and retaining the insulator; and 
 a ground electrode having a base end portion welded to the metal shell and a distal end portion facing an axially front end portion of the center electrode with a gap left between the distal end portion of the ground electrode and the front end portion of the center electrode, 
 the ground electrode including a surface layer defining a surface of the ground electrode and a core located inside the surface layer and having a higher thermal conductivity than that of the surface layer, 
 the surface layer having a thickness of 0.2 mm to 0.4 mm at a specific position that is located 1 mm from the base end portion in a direction toward the distal end portion along an outer shape of the ground electrode, 
 wherein the spark plug satisfies the following condition: W1≧W2×1.55−(W3+0.25) where W1 (mm) is a width of the metal shell at a weld region of the metal shell joined with the base end portion in a specific direction that extends perpendicular to the axis direction through a center line of the ground electrode; W2 (mm) is a thickness of the ground electrode at the specific position in the specific direction; and W3 (mm) is a thickness of the surface layer at the specific position in the specific direction. 
 
     
     
       2. The spark plug according to  claim 1 ,
 wherein the core includes a first core member located on an inner side thereof and a second core member located on an outer side thereof so as to circumferentially surround the first core member and having a higher thermal conductivity and a lower hardness than those of the first core member; 
 wherein the first core member has a protruding shape that protrudes to a rear of the second core member in the axis direction; 
 wherein the weld region has an undulating shape that undulates according to the protruding shape of the first core member; and 
 wherein a distance between frontmost and rearmost ends of a part of the weld region in contact with the first core member in the axis direction is 0.15 mm or more. 
 
     
     
       3. The spark plug according to  claim 1 ,
 wherein the core includes a first core member located on an inner side thereof and a second core member located on an outer side thereof so as to circumferentially surround the first core member and having a higher thermal conductivity and a lower hardness than those of the first core member; and 
 wherein, when viewed in cross section along a plane defined by the axis direction and the specific direction, the spark plug satisfies the following condition: W4/W5≦0.34 where, on the assumption that: a first imaginary line extends in parallel to the specific direction through a midpoint of a spark gap SG, which is defined between the center electrode and the ground electrode in the axis direction; and a second imaginary line passes through an intersection point of the first imaginary line and a plane of the ground electrode located closer to the center electrode and intersecting the first imaginary line at an elevation angle of 45 degrees toward the specific direction, W4 is the sum of widths of the second core member on the secondary imaginary line; and W5 is the sum of widths of the surface layer on the second imaginary line. 
 
     
     
       4. The spark plug according to  claim 2 ,
 wherein, when viewed in cross section along a plane defined by the axis direction and the specific direction, the spark plug satisfies the following condition: W4/W5≦0.34 where, on the assumption that: a first imaginary line extends in parallel to the specific direction through a midpoint of a spark gap SG, which is defined between the center electrode and the ground electrode in the axis direction; and a second imaginary line passes through an intersection point of the first imaginary line and a plane of the ground electrode located closer to the center electrode and intersecting the first imaginary line at an elevation angle of 45 degrees toward the specific direction, W4 is the sum of widths of the second core member on the secondary imaginary line; and W5 is the sum of widths of the surface layer on the second imaginary line. 
 
     
     
       5. The spark plug according to  claim 1 ,
 wherein, when viewed in cross section parallel to a plane defined by the axis direction and the specific direction, a center line of the ground electrode extending at the specific position in the axis direction is located closer to the center electrode than a center line of the metal shell extending at the weld region in the axis direction. 
 
     
     
       6. The spark plug according to  claim 1 ,
 wherein, when viewed in cross section parallel to a plane defined by the axis direction and the specific direction, the metal shell has a protruding shape that protrudes at the center thereof toward the front in the axial direction, and the core has an undulating shape that undulates according to the protruding shape of the metal shell, and an outer thickness of the ground electrode in the specific direction at axially rear end points of the core is larger than a thickness of the distal end portion of the ground electrode. 
 
     
     
       7. The spark plug according to  claim 2 ,
 wherein, when viewed in cross section parallel to a plane claimed by the axis direction and the specific direction, a center line of the ground electrode extending at the specific position in the axis direction is located closer to the center electrode than a center line of the metal shell extending at the weld region in the axis direction. 
 
     
     
       8. The spark plug according to  claim 2 ,
 wherein, when viewed in cross section parallel to a plane defined by the axis direction and the specific direction, the metal shell has a protruding shape that protrudes at the center thereof toward the front in the axial direction, and the core has an undulating shape that undulates according to the protruding shape of the metal shell, and an outer thickness of the ground electrode in the specific direction at axially rear end points of the core is larger than a thickness of the distal end portion of the ground electrode. 
 
     
     
       9. The spark plug according to  claim 3 ,
 wherein, when viewed in cross section parallel to a plane defined by the axis direction and the specific direction, a center line of the ground electrode extending at the specific position in the axis direction is located closer to the center electrode than a center line of the metal shell extending at the weld region in the axis direction. 
 
     
     
       10. The spark plug according to  claim 3 ,
 wherein, when viewed in cross section parallel to a plane defined by the axis direction and the specific direction, the metal shell has a protruding shape that protrudes at the center thereof toward the front in the axial direction, and the core has an undulating shape that undulates according to the protruding shape of the metal shell, and an outer thickness of the ground electrode in the specific direction at axially rear end points of the core is larger than a thickness of the distal end portion of the ground electrode. 
 
     
     
       11. The spark plug according to  claim 4 ,
 wherein, when viewed in cross section parallel to a plane defined by the axis direction and the specific direction, a center line of the ground electrode extending at the specific position in the axis direction is located closer to the center electrode than a center line of the metal shell extending at the weld region in the axis direction. 
 
     
     
       12. The spark plug according to  claim 4 ,
 wherein, when viewed in cross section parallel to a plane defined by the axis direction and the specific direction, the metal shell has a protruding shape that protrudes at the center thereof toward the front in the axial direction, and the core has an undulating shape that undulates according to the protruding shape of the metal shell, and an outer thickness of the ground electrode in the specific direction at axially rear end points of the core is larger than a thickness of the distal end portion of the ground electrode. 
 
     
     
       13. The spark plug according to  claim 5 ,
 wherein, when viewed in cross section parallel to a plane defined by the axis direction and the specific direction, the metal shell has a protruding shape that protrudes at the center thereof toward the front in the axial direction, and the core has an undulating shape that undulates according to the protruding shape of the metal shell, and an outer thickness of the ground electrode in the specific direction at axially rear end points of the core is larger than a thickness of the distal end portion of the ground electrode.